This paper presents a two-dimensional simulation model of an amoeboid robot that exhibits locomotion in a decentralized manner. The significant feature to note is that the model does not control friction between parts of the robot and ground explicitly but exploits passive dynamics of the inner fluid of the robot, i.e., thixotropic flow, in order to generate locomotion. Thixotropy is a very interesting rheological property of a fluid to form a gelled structure over time when not subject to shearing and then to liquefy when agitated, which is observed in protoplasmic streaming of amoeba and plasmodium of true slime mold. Simulation results show that embedding this passive dynamics induces morphological positive feedback mechanism, leading to convection of the inner fluid, which in turn generates locomotion without relying on any hierarchical structure. The results obtained are expected to shed new light on revealing the secret of how decentralized control should be designed.